1. Field of Invention
The present invention relates to a method of forming a metallic fuse. More particularly, the present invention relates to a method of forming a metallic fuse that demands a lower laser power for circuit repair.
2. Description of Related Art
In the manufacturing of semiconductor memories, product yield is of great importance. If there is one or more defective memory cells in a semiconductor memory product, the finished product as a whole is defective and has to be scrapped or repaired. As the level of device integration increases. the number of defective memory cells also increases proportionately. In other words, as the level of integration of semiconductor memory devices increases, product yield is correspondingly lowered.
As advanced techniques for fabricating semiconductor memory devices develop, the packing density of memory devices increases considerably. Consequently, fabrication becomes increasingly difficult and complex. Since impure particles (or fragments) appear when the memory devices are produced, a lower yield rate for highly integrated circuits is unavoidable. Hence, to increase the yield rate of circuit fabrication, redundant backup devices are often added to boost the product yield of conventional operations. Besides memory cell array for storing binary bits, an additional backup memory cell array is often provided so that defective memory cells can be easily replaced. Each redundant memory cell is connected to a word line and a bit line, respectively. Thus, even if more than a thousand defective memory cells are found after inspection, the defective memory cells can be replaced by the redundant memory cells in the backup array. Consequently, a defect-free memory array can be obtained on a silicon chip.
The above scheme of using redundant elements in memory circuits has the advantages of a higher yield and defect-free circuits. However, in exceptional cases, when the number of defective elements is higher than the number of redundant elements provided, even this scheme does not work.
In general, the above scheme is implemented by forming a redundant memory cell array around a main memory cell array when the semiconductor memory devices are manufactured. The main memory cell array and the redundant memory cell array are normally linked by polysilicon fuses, which can be melted away by a laser beam or a high current. When a defective memory cell needs to be fixed, the protective fuse is broken by a laser beam or an electric current. On the other hand, if the defective cell does not require fixing. the protective fuse can stay as it is.
FIGS. 1A and 1B are schematic, cross-sectional views showing the progression of manufacturing steps in forming a conventional polysilicon fuse.
First, as shown in FIG. 1A, a local oxidation of silicon (LOCOS) method is used to form a field oxide layer 12 over a semiconductor substrate 10. Then, the field oxide layer 12 is patterned to form an active region 13. Next, a polysilicon fuse structure 14 is formed above the field oxide layer 12. Thereafter, an inter-metal dielectric layer 16 is formed covering the entire substrate structure. including the polysilicon fuse 14. Subsequently, photolithographic and etching techniques are used to form an opening 17 in the inter-metal dielectric layer 16 that exposes a portion of the active region 13. Next, a conductive plug 18 is formed inside the opening 17, and then a layer of conductive material is deposited over the inter-metal dielectric layer 16 and makes electrical contact with the conductive plug 18. The conductive material is patterned to form a conductive layer 20.
Similarly, using the above method, another inter-metal dielectric layer 22 is formed over the conductive layer 20. Next, photolithographic and etching operations are again used to form another opening 24 in the inter-metal dielectric layer 22 and expose a portion of the conductive layer 20. Thereafter, another conductive plug 26 is formed inside the opening 24, and then a conductive material is formed over the inter-metal dielectric layer 22 and makes electrical contact with the conductive plug 26. The conductive material is patterned to form a conductive layer 28.
Next, a chemical-vapor deposition method is used to form a silicon nitride layer over the entire substratte structure that includes the conductive layer 28. The silicon nitride layer 30 serves as a protective layer in subsequent operations. Thereafter, conventional photolithographic and etching methods are used to pattern the silicon nitride layer 30 so that an opening 32 is formed in the silicon nitride layer 30. The opening 32 exposes a portion of the inter-metal dielectric layer 22. Moreover, the opening 32 is formed in a location vertically above the polysilicon fuse structure 14.
Next, as shown in FIG. 1B, when a defective memory cell needs to be reinstated, the polysilicon fuse 14 of that particular cell has to be cut by a laser beam. Consequently, the defective memory cell is replaced by a redundant memory cell. In the laser cutting operation, the laser beam penetrates the opening 32 and passes through the inter-metal dielectric layers 16 and 22 to reach the polysilicon fuse 14, deep below, hence cutting open the luse connection and achieving the necessary defective cell replacement. Since the laser beam's power is very high, portions of the inter-metal dielectric layers 16 and 22 penetrated by the laser beam sublimate and turn into gas. Ultimately, a deep opening 34 that exposes the field oxide layer 12 is be formed when the memory cell restoration is completed.
However, in the conventional method of fabricating polysilicon fuses, the fuses are still laid on top of the field oxide layer even though there are more than three intervening inter-metal dielectric layers. Since the polysilicon fuses are too deep below the surface, the laser beam's power must be very high in order for it to carry out the reparation. Furthermore, when the polysilicon fuses are too deep within the structure, it is difficult for the laser beam to reach the fuse without part of the laser beam being dispersed. Hence. much laser power is wasted and yield of the reparation is low.
In practice, when the laser power used in burning a polysilicon fuse is between 2.times.10.sup.-6 to 3.times.10.sup.-6 joules/sec, the rate of repair is 50% at most. If a higher rate of repair is desired, the laser power output has to be increased to an even higher value. However, turning up the laser power can easily damage part of the silicon wafer.
In light of the foregoing, there is a need to provide a method of forming a polysilicon fuse that requires less laser power to achieve the desired result.